The NX bit is not just about security (purplesyringa.moe)

72 pointsby torutofu3 days ago36 comments

repiret 6 hours ago

"Speculative instruction fetch to device memory crashes device" is in my list of favorite bugs ever found. I had ran into it on a Cortex-M4 without an MPU, so I couldn't even fix the memory attributes, the only solution was to gerrymander the compiled code to prevent the speculative fetch.

It's crazy that Armv8/9-A permits speculative instruction fetches to device memory. Crazy enough that I had to look it up to believe it:

"Hardware does not prevent speculative instruction fetches from a memory location with any of the Device memory attributes unless the memory location is also marked as execute-never for all Exception levels." - ARM DDI 0487K.a § B2.15.2

The Armv7-M spec is less clear. It does say: "The architecture does not permit speculative accesses to memory marked as Device," in contrast to Armv8/9-A which qualifies a similar statement with "data accesses". But then it later says "To ensure correctness, read-sensitive locations must be marked as non-executable". (This is all from ARM DDI 0403E.e § A3.5.7)

I don't have v7-A older handy to compare what they say.

asveikau 7 hours ago

On this theme of it not being just about security: if you have a bug like a use after free and it happens to cover a function pointer, the nx bit can ensure that when you follow that pointer through a call, you get a clean trap as close to the failure point as possible. If it blindly executed stale bytes as code, maybe the crash and stack trace doesn't look as nice.

But then, a lot of correctness bugs like that are also security problems.

anyfoo 4 hours ago

Nowadays, it's considered bad form to map anything at 0... you now know one of the reasons why!

In the case that I know that has a 1:1 identity mapping of the address space, the physical address space doesn't actually have anything at 0 either, so it still doesn't need to be mapped.

sleirsgoevy 2 minutes ago

Post author here. I wanted the code to be "as generic as possible", so I had to accept the possibility of some useful thing being at address 0.

mubbicles 7 hours ago

I appreciated this article. I've asked our CSP guy the difference, but this helps me better understand device vs ns. I also appreciate that this doesn't appear AI written.

eqvinox 7 hours ago

Honestly feels like a misdesign in ARM. Where does it ever make sense for Device memory to not be data prefetchable, but allow instruction prefetch? It should IMHO disable all prefetch…

achierius 6 hours ago

(Total guess) this feels like an attempt at restricting what parts of the hardware pipeline need to know what; if prefetches need to key off of both the device bit and the nx bit, then both of those need to be piped into the frontend, whereas if device is only relevant for data prefetches then only the nx bit needs to be available there (while device would be piped into the backend where the data prefetcher lives).

adrian_b 6 hours ago

Not really.

To the front end, one needs to pipe only appropriate logical combinations of the flags that describe the memory properties, not the flags as they are stored in some control register.

So a control register should contain flags that are meaningful for the programmer, while the kind of flags that hardware happens to need can be generated with a few logic gates from them and routed through hardware wherever they are needed.

It makes more sense to classify the memory in a few types, which must be specified by the programmer, including a "device memory" a.k.a. "memory-mapped peripherals" type, instead of having to specify for each memory area a long set of attributes about each kind of access that may be allowed, or not.

This is how it is done in x86-64.

phire 6 hours ago

The problem is that unlike data prefetch, the so-called "instruction prefetch" is not actually prefetch at all.

It's simply speculative execution. Which doesn't look any different to regular execution. The fetcher has no idea that its predicted branch is about to invalidated and flushed, otherwise it would never have issued that fetch.

Actually, on a modern OoO core, [0] it's very rare for the instruction fetcher to not be doing speculative fetches. Even when it's not predicting a branch, the fact that it has "predicted" the lack of a branch is speculative in itself. It assumes it didn't fetch a branch in the last cycle, but it can't be sure until after instruction decoding, which takes at least 2 cycles (more on larger L1i caches).

About the only time the instruction fetcher is not doing speculative fetching is for a single cycle after each miss-predicted branch.

[0] Or even something technically in-order, like the Cortex A53 cores here. They might issue in-order, but because of how they implement dual issue, they look somewhat close to a simple OoO core... I suspect they actually do register renaming. And (most importantly) importantly they have a branch predictor.

repiret 5 hours ago

No, the problem really is the prefetch itself when there are undesirable side-effects if the bus sees that memory read.

phire 5 hours ago

Well yes. That is why the "prefetch" is a problem.

But the original question was asking why disabling data prefetching to a memory region didn't automatically disable instruction prefetching at the same time.

And the answer is that speculative execution is a completely different mechanism that I'm not even sure can be disabled, at least not per memory region.

renox an hour ago

Why do you say this? The post is quite clear that marking the memory region as NX fix the issue caused by speculative execution.

tripdout 8 hours ago

I really wish I understood this, it hits a bunch of topics that I've heard of and are/sound interesting, but I don't know enough to follow it. I don't get the link between the NX bit (which I get) and speculative access.

zephen 7 hours ago

In an attempt to go fast and beat benchmarks and other computers, CPUs attempt to speculatively execute code, and then later undo the results of the speculation if it turns out it was wrong. This causes all sorts of security issues (spectre, meltdown, and friends et al.) even when it's done relatively competently.

When it's done incompetently as on this ARM implementation, then you can't even run perfectly good and correct code, because the CPU will attempt speculative execution on a location that you never asked it to execute code at, and then bork itself when it realizes that can't possibly work.

Naturally, this is the sort of problem that requires tedious dissection of what exactly happened, and copious amounts of alcohol.

MBCook 6 hours ago

So in this specific case, if I understood correctly, here’s what should happen:

Interrupt(?) fires to trigger hypervisor, hypervisor figures out what it needs to do, jumps to that code, does its job, returns.

The “figured out what it needs to do” is the issue right? So what was actually happening was:

Same start… CPU predicts what hypervisor will do, speculatively loads instructions from mispredicted branch target, that wrong instruction reads memory(?) against the “no data prefetch” settings for that part of memory, CPU blows up/halts/whatever.

The fix is to mark the area the branch was mispredicted to in such a way that the CPU won’t prefetch instructions. Thus that won’t be run and prefetch data, thus no violation. CPU execution continues taking the correct branch and everything is fine.

repiret 6 hours ago

Speculative instruction fetches fetch code from "a location that you never asked it to execute code at" by design. If it already knew you asked it to execute code there, it wouldn't be speculative.

The Armv8/9-A architecture reference manual is clear that speculative instruction fetches are permitted in Device memory unless that memory is also marked NX. So if your hardware has side-effects from a certain address, but it maps it as Device non-NX memory, then your code is not "perfectly good and correct". Assigning correct memory attributes is one of the many things needed for correct code.

RiverCrochet 7 hours ago

Basically, the NX bit prevents CPU behavior (speculative fetches) that had a hand in Spectre-type vulnerabilities. That's surprising because that's not its purpose. This is for ARM CPUs.

jnwatson 7 hours ago

No, NX precedes Spectre by a long shot. It was originally intended so an attacker couldn't use a buffer overflow to change the PC and execute directly out of the attacker-controlled buffer.

adrian_b 6 hours ago

That is exactly what the poster to whom you replied said.

So Arm did not add another means to disable this kind of speculative execution, after Spectre was discovered, but they just reused the existing NX flag, expanding its functionality.

Nail2680 6 hours ago

NX means not executable, so some bytes are excutable, some are non excutable. so some are instructions for the processor and others are data. Sometimes those are mixed and the cpu pukes out a bunch and restarts interpretation. most of the time those are normal. keep it normal and go fast.

dmitrygr 7 hours ago

This is doubly weird because actually purposefully executing from device memory is not allowed: “Trying to execute code from a region marked as Device is UNPREDICTABLE.” So: can’t reliably execute from there but can speculatively instruction fetch from there. Funsies!

https://support.arm.com/documentation/100941/0101/Memory-typ...

achierius 7 hours ago

> while ARM does provide a reference implementation of the architecture, vendors are free to customize it at will, or even roll their own implementations

A nitpick, but this is only true for some vendors, depending on their license, and is very much company-by-company. Many vendors, even big names like Meta, don't have the ability to roll their own. And even for the ones who do, 'customize at will' is a bit strong, as ARM very much does want to maintain uniformity across userspace implementations. E.g. Nvidia shouldn't add new traps for architecturally-legal behavior, since then code compiled for Apple hardware wouldn't work on Grace. Or worse, not trap for architecturally-illegal behavior, since then code compiled for Grace might not work for anyone else at all!

adrian_b 6 hours ago

TFA contains a reference to the Linux kernel, where there is a workaround for a quirk in the Apple CPUs, where in hypervisor mode they diverge from the official Aarch64 specification.

So by "vendors" it was indeed meant "some vendors" who can afford to not care much about compatibility with the specification.

achierius 6 hours ago

Unfortunately I can't comment much on Apple's situation since I work there :)

Regardless what I mean to convey is that ARM would prefer such deviations to be rare, and especially for them to not be visible from userspace. Deviations will always exist if only due to hardware bugs, so contracts can only do so much.

my123 6 hours ago

It's an old comment.

iirc it's a documented feature now - FEAT_E2H0, https://support.arm.com/documentation/109697/2025_12/Feature...

And it was retroactively defined to be allowed starting from Armv8.0.

Apple designs pre-date the ID register bit for it being a thing so it takes a quirk there however.

crazy1_ 8 hours ago

this is so amazing

Nail2680 8 hours ago

Hot take: NX bit is shit W^X is shit. proper JIT is having objects written as needed, and cache line flushing is full bullshit, we need self modifying code as a first class citizen and with modern techiques we can have it work and not be crazy slow, it is currently cuz shits fucked, but we can do better.

Nail2680 7 hours ago

Well it was a hot take.

eqvinox 7 hours ago

Are you unaware that you can write code to RW memory, remap it to RX and then run it? That's how all JIT works these days.

(Or did you confuse cache flushing with TLB flushing? The remap does the latter, not the former.)

Nail2680 7 hours ago

Yeah, that works okay, but really is a you shouldn't, with pipelining you lose all the predictive decoding. The Synthesis kernel did some cool ass shit with this, but failed in other architectures due to pipelining, which speeds up shit, but change the opcodes(with other instructions(ie selfmodyfing code)) and shit gets flushed.

RiverCrochet 7 hours ago

L take. RAM is too slow for self-modifying code to be anywhere but F tier. This isn't 6502 land.

Nail2680 7 hours ago

But it could be, there are bits about doing computation in RAM, self modifying code could work.

neerajsi 7 hours ago

Disallowing smc is a significant perf/power win. For cpus that run a large variety of large code (e.g. a web browser or ux stack), being able to cache a large instruction footprint and fetch/decode it quickly is important. Having to have the icache snoop data writes and entangle the i-fetch with the store buffer machinery would be a huge penalty to pay for a niche use case. Unlike loads, which are a small fraction of instructions to disambiguate with stores, you'd have to disambiguate every single instruction.

JIT is important to Apple platforms, and they seem to manage to make it work well enough even with the need for explicit invalidation.

Nail2680 7 hours ago

So that is only true because we do it, there is a world where we optimize differently and that self modifying version works better, reread the synthesis kernel thesis(one, it is super easy, two they did this), we could have hardware that does this. Because we don't have hardware that does this we don't

achierius 7 hours ago

FWIW if you look at WebKit/JavaScriptCore, the trend has been towards less-frequent code modification, e.g. inline caches are mostly no longer repatched inline. Whole-function reoptimization is still worth the overhead of sys_icache_invalidate() + `ISB`'ing everyone involved, but at smaller granularities not so much.

Nail2680 6 hours ago

I love you, keep it up dude. Exactly the kinda shit I would say if I didn't want to lose more fake internet points now.